The present invention relates to a process for the continuous manufacture of foamed sheets of thermoplastic material, in particular polyvinylchloride, wherein the sheets are characterized by an extreme width to thickness ratio and a small thickness tolerance.
In the continuous production of foamed plastic sheets one differentiates basically between the method of "foaming outwards" and that of "foaming inwards".
The method of "foaming outwards" is the older, simpler method in which the thermoplastic plastic material, already containing the foaming agent, is extruded through wide, slit-shaped nozzles. The hot, soft, still-foaming extrusion material is led directly to an in-line calibrating facility where the process of expansion of the foam is stopped by the contact with the cooled surface.
The process of "foaming inwards" is described in German published patent application No. 17 29 076 for the Celuka process. In this process a specially designed metal core, a so-called torpedo, is employed in the extrusion die to produce a hollow space inside the extruded material, the outer surfaces of which are cooled by a closely following, in-line calibrating facility. This prevents the skin of the extruded material from foaming with the result that, initially, a hollow section is produced. The still hot inner zone which has a low viscosity foams towards the center until the space inside has been filled and counter pressure has been built up.
Both of the described processes can produce thin sheets but only in relatively small widths thereby considerably limiting the use of the sheets produced by said processes.
In the "foaming outwards" process the thickness of the skin formed depends on the intensity of cooling and pressure on the cooling surface. However, there is a certain minimum skin thickness which must be provided as, due to the cooling on the walls, the pressure in the cells falls and, when there is too little pressure inside, the cells collapse thereby affecting the quality of the surface. Also, in particular when vacuum calibration is employed, the skin has to provide a certain supporting function to prevent changes in structure and shape taking place due to frictional forces which are created as the sheet or strip is pulled through the calibrating unit.
Although this process does not require such expensive dies as the inward foaming process, and also allows a higher production rate, the quality of the foamed product is not completely satisfactory. Apart from the fact that the surface is not always smooth, the sheet varies greatly in thickness.
Although in the inward foaming process the density of the foamed material can be varied by appropriate choice of parameters such as torpedo shape and size, cooling rate and withdrawal rate, composition etc., it is unavoidable that the outer skin, consisting of non-foamed material, is relatively thick, and the foamed core layer contains pores widely varying in size, the pore diameter increasing across the section towards the middle of the core layer. Often the structure is nonhomogeneous, in particular in the cross section at the center line in the core where the two layers foaming from the outside towards the center meet. Not infrequently a laminar, linear texture, or even internal separation of the material can be observed at the center line. The resultant weakening of the core can be of disadvantage in applications where the sheet is subject to high mechanical stressing and rigidity is especially important, even though the rigidity of such sheet material depends essentially on the non-foamed outer skin. In applications where mechanical strength is of only minor importance e.g. for thermal insulation, the sheet material produced by the inwards foaming process is uneconomic as the relatively thick non-foamed outer skin contains a substantial amount of the plastic, which is therefore not efficiently used. A further, very important disadvantage of this process is, however, that wide, thin sheets--over ca. 1000 mm wide and less than 10 mm thick--cannot be produced by this process or only at great expense. There are two reasons for this. First, it is not possible to create the space in the die with a torpedo which has a very small height to width ratio, and secondly the outer skin cannot be produced thinner than a certain value due to the intensive cooling needed to create the hollow section during calibration. It is therefore not possible by the present state of the art to produce by this process plastic sheets which are less than 10 mm thick and 1000 mm or more wide. Also, when it is possible to manufacture extrusion dies to make sheets less than 10 mm thick and over 1000 mm wide, the production costs are so high that the economic use of such sheets is questionable.
In view of the disadvantages of the processes for manufacturing foamed plastic sheets described above, the inventor set himself the task of developing a process for the production of broad, thin foamed plastic sheets, in particular sheets made of polyvinylchloride having a homogeneous cell structure, dense surface, especially such which have a thin surface skin and at the same time only small variation in thickness.